Fine Geometry Solar Cell

ABSTRACT

A fine geometry solar cell having a top surface contact comprising substantially more and finer metallic fingers spaced close together for collecting photocurrent. Junction depth and/or impurity concentration may be reduced significantly. The method for making the fine geometry solar cell, comprises in ordered steps, the processes of diffusion, oxidation, photolithography, metallization and plating.

United States Patent Lindmayer Reissued Nov. 11, 1975 [54] FINE GEOMETRYSOLAR CELL 3565.686 2/I97l Bnhcock 1. l36/89 X 3.589 4 Q l 1 1 H I36 89[75] Inventor: Joseph Lindmayer, Bethesda. Md. 9 6 6H 7 Tqmqd at I] I,

[73] Assignee: Communications Satellite OTHER PUBLICATIONS Commit),Washington, D13 M. Wolf Proceedings Of the IRE July 1960 pp. 1246, [22]Filed: Nov. 19, 1974 1 2 v Technical Report AFAPL-TR-GS-S 2/1965 Re-[21] Appl' 52512l search on Thin Film Polycrepballire Solar Cells byRelated U.S. Patent Documents Aldrich fil I25 I26 PP- I29, I33 Reissueof:

[64] Patent No.: 3,811,954

I d; M 21, 1974 Primary E.\'am1'nerAllen B. Curtis A 1 NO 134393Attorney, Agent. or Firm-Sughrue. Rothwell. Mion. Filed: Sept. 28, 1971Zinn & Macpeak I52] [51] Int. CI.- HOIL 31/02; HOIL 31/04 [58] Field ofSearch 136/89, 572 A fine geometry Solar having 11 1 Surface 999K119!comprising substantially more and finer metallic fin 5 References Citedgers spaced close together for collecting photocurrenti UNITED STATESPATENTS Junction depth and/or impurity concentration may be a reducedsignificantly. The method for making the line f gi ag 3 geometry solarcell, comprises in ordered steps the 361694 1/1968 nsfflaii'lliiIIIlIIlIIliIIIIIIIlIQ. 136/89 of diffusion. oxidation. mphy.3,411I9s2 11/1968 ROSS et =11. .6 l36/S9 1 metallllauon and P 3.472.69810/1969 Mandelkorn 11 l36/89 1 3,493,822 2/1970 1165 136/89 x 18 Clams 4Drawmg F'gmes /NEW 60 FINGER GEOMETRY lO/o- PRESENT a 5i CELLS STANDARD6 FINGER O\ GEOMETRY E o 6 5 CONVERSION EFFICIENCY CONVERSION EFFICIENCYLIMITED E LIMITED BY SERIES BY DEEP DIFFUSION AND M RESISTANCEASSOCIATED LATTICE DAMAGE D' lo c61 SURFAOEOONCENTRATION OF DIFFUSEDLAYER EFFICIENCY /o Reissued Nov. 11, 1975 LIGHT pyp FIG. 2

STANDARD GEOMETRY (6 FINGERSI BUSBAR FIG. 3

FINE GEOMETRY I60 LINES) BUSBAR SURFACE CONCENTRATION OF DIFFUSED LAYERFINE GEOMETRY SOLAR CELL Matter enclosed in heavy brackets appears inthe original patent but forms no part of this reissue specification;matter printed in italics indicates the additions made by reissue.

BACKGROUND OF THE INVENTION This invention relates to solar cells. andmore particularly. to a fine geometry solar wherein the surface throughwhich light enters comprises a substantial number of very fine metalliclines (or pattern) which collect current.

The use of photovoltaic devices. commonly known as solar cells, whichconvert light energy to useful electrical energy is well known. Lightentering these solar cells is absorbed. thereby generating electron-holepairs which are then spaeially separated by the electric field producedby the solar cell junction and are eollected at respective top andbottom surfaces of the solar cell. For example, in an n-p type solarcell electrons will travel to the top surface where they will then becollected by a metallic grid positioned thereon. The metallic grid maytypically comprise six metallic fingers separated along the top surfaceby a relatively large distance and connected to each other by a commonbus bar. The electrons will travel either directly to the metallicfingers or approach the top surface between the fingers and then travelalong the surface of the solar cell until they can be collected by oneof the fingers. Holes. on the other hand, will travel to the bottomsurface of the solar cell where they may be collected by a metallicsheet covering the entire bottom surface.

The six-fingered metallic grid is necessary at the top surface of thesolar cells in order to enable light to enter the solar cell. However.one problem associated with the six-fingered construction relates to therelatively large separation between the fingers. Electrons which musttravel along the surface to the metallic fingers encounter a highsurface resistance. Therefore. due to the relatively long distance theelectrons must travel before collection, and due to the problem ofsurface resistance. a series resistance may develop. thereby limitingthe efficiency (electrical power out put/solar power input) of solarcells by limiting the electrical power output.

The prior art has sought to obviate the above problem by diffusing animpurity into the surface of the solar cell in a higher orderconcentration, on the average of about atoms per square centimeter orhigher. Higher order concentration (i.e., heavier diffusion) lowers thesurface resistance but introduces other problems. Higher orderconcentration of impurities is obtained by a process known as solidsolubility diffusion." i.e.. the solar cell is allowed to assume as manyimpurities as it can on the surface. e.g.. approaching 10* atoms percubic centimeter. However, in such a solid solubility process there iscrystal lattice damage to the solar cell which propagates deep into thesolar cell substrate. The efficiency of the solar cell is thereby reduced in two ways. First. the damage to the crystal lattice causes areduction in the diffusion length or life time of minority carriers.This means that holes. for example. in an n-type diffused region willrecombine with available electrons before they can be separated by thejunction. Secondly, as discussed below. damage to the crystal structureaffects the power output of the solar cell (which is basically a diode)by softening' the current(i)-voltage(v) characteristics of the diode. lnaddition. the diffusion of such higher order concentration of impuritiescreates a relatively deep junction ofabout 4.000 A. This relatively deepjunction means that light of relatively short wavelengths (where solarenergy peaks) cannot penetrate beyond the junction. but is absorbed inthe diffused region He. between the top surface and the junction).Electron-hole pairs generated in the diffused region have a relativelyshort diffusion length (even if there were no crystal lattice damage)and therefore will largely recombine before separation by the junction.

The present invention has the advantage of improving the efficiency ofsolar cells in the short wavelength. i.e.. blue-violet portion of thespectrum corresponding to 0.3-0.5 microns thereby sharply increasingoutput power. The present invention also has the advantage of enabling adegree offreedom in the design of solar cells by reducing the junctiondepth and/or reducing the impurity concentration while improving solarcell efficiency. in addition. the effect of radiation damage to thesolar cell is decreased with improvement in effi ciency in the shortwavelength region. Also. the use of specified metals for the metalliccontact of the present invention provides a moisture resistant contact.

An advantage of extremely shallowjunctions wherein there is no crystallattice damage is that diodes become much more ideal. in the simplejunction theory or socalled "diffusion theory" the following relationapplies:

where l is the diode current. l,, is the reverse diode cur rent. V isthe applied voltage and kT/q is the thermal voltage. Actual solar cellsdo not follow this relationship. but rather the following:

where n is a quantity greater than unity. ln conventional solar cells, n2 while of course in the ideal case. n I. This fact "softens the l-Vcharacteristics of solar cells. As expressed in terms of the fill factorF=actual power to load/short circuit current X open circuit voltageDefined in this fashion. conventional solar cells show an F of about 72percent. With the extremely shallow diffusion and reduced impuritysurface concentration practiced by the present invention as describedbelow. an 11 value of about l.] and F approaching percent may beobtained. These numbers represent an almost ideal junction.

SUMMARY OF THE INVENTION The invention comprises any type of solar cell.e.g.. silicon or gallium arsenide. having a top surface currentcollector comprising a significantly greater number of fine metallicfingers (or other fine geometric pattern) wherein the physicalseparation between the fingers and the width of each finger issubstantially reduced. The junction depth and/or impurity concentrationis reduced in accordance with the degree of freedom provided by the useof the fine geometry cell. The solar cell is made by first introducingimpurities into. for exam ple. a silicon slice. and then oxidizing thesolar cell. Then. the fine metallic pattern is placed on the top surfaceof the solar cell using a photolithography tech nique. Finally. aplating process is used to build up the fingers of the metallic patternto a proper thickness.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. I is a general block diagram of aside view of a solar cell having metallic fingers located on the topsurface.

FIG. 2 is a diagram on the standard geometry six-fingered contact usedon the top surface of the solar cell of FIG. I.

FIG. 3 is a diagram of the fine geometry metallic contact of the presentinvention used on the top surface of the solar cell of FIG. 1.

FIG. 4 is a graph of efficiency vs. surface concentration of diffsuedlayer for a silicon solar cell comparing the prior art six-fingeredgeometry with the fine geometry of the present invention.

DETAILED DESCRIPTION OF THE DRAWINGS Referring to FIG. I there is showna side view of a typical solar cell. There will be described a singlecrystal. n-p silicon solar cell though the invention has applicabilityto all types of single crystal solar cells including. for example. GaAssolar cells. The term single crystal" is well known in the art andrefers to lattices having absolute perfect crystallographic order. butas described herein. also includes nearly single crystal cells which arealmost perfectlycrystallographic. In ad dition the inventive conceptsare not limited to single crystal solar cells but may be applied to thinfilm solar cells.

The single crystal silicon solar cell comprises a silicon substrate 1 ofp-typc material and a silicon layer 2 of n-type material with an n-pjunction 3 positioned a predetermined depth below the top surface ofsilicon layer 2. In an n-p silicon solar cell the junction 3 willproduce an electric field directed towards the substrate 1 therebyresulting in generated electrons flowing to the top of surface 2 withholes flowing to the bottom of substrate 1 wherein the holes may becollected by a contact 4 covering the entire back of the bottom surfaceof layer I.

The metallic grid pattern 5 used for collection of the electrons flowingto the surface through which light en ters is positioned on top ofsilicon layer 2. In the prior art solar cells the grid pattern 5 maycomprise a six-fingcred metallic contact ofa type shown in FIG. 2. For asolar cell having dimensions 2 X 2 cm. each metallic finger isapproximately 0.30 centimeters apart with each finger having a width ofabout 300 microns. The entire metallic grid would block between 8-l0percent ofthe light falling on the solar cell. The metallic grid of thepresent invention. however. as seen in FIG. 3. comprises for a 2 X 2 cmsolar cell approximately 60 metallic fingers wherein the separationbetween each finger is approximately 0.03 centimeters with each metallicfinger being between 1-20 microns in width. The fine geometryconfiguration of the present invention would block less than l0 percentof the solar light. The fine metallic fingers may lie parallel to themain busbar and be connected thereto by tapered, intermediate buses asshown in FIG. 3. or alternately the fine metallic fingers may all lieperpendicular to the main busbar and be directly connected thereto. inthe manner shown in FIG. 2.

With the fine geometry solar cell it is now possible to lower theimpurity concentration and/or decrease junction depth. If the junctiondepth is decreased (cg. by shortening diffusion time and/or loweringdiffusion temperature) the efficiency will be improved in three ways.First. more short wavelength light will penetrate beyond the junction 3to the p-type silicon substrate 1 to generate electron-hole pairs insubstrate 1. Electronhole pairs generated in substrate 1 have a longerlifetime than electron-hole pairs generated in n-type layer 2. Secondly.though all the electrons generated in the solar cell will encountergreater surface resistance at the top of layer 2 with a lowering ofsurface impurity concentration. the distance along the surface needed tobe traveled by the electrons prior to collection will be greatlyreduced. In the alternative. if the surface concentration is loweredwithout decreasing junction depth. the resistance encountered by theelectrons will again be offset by the fine geometry contact of FIG. 3.Also. in the latter case. though most of the short wavelength light willgenerate electron-hole pairs in diffused layer 2. due to a loweringofthe impurity concentration in layer 2 the lifetime of holes will beincreased.

The manner in which the n-p. silicon solar cell of the present inventionhaving a reduced junction depth is made will now be described. First ap-type silicon piece is cut and polished into a slab. for example. 2 X 2cm. Then. n-type impurities. c.g.. any of the elements from Group VA ofthe table of elements. such as phosphorus. arsenic or antimony. arediffused into the p-type substrate forming an np junction. Whereas theprior art has a diffused junction depth of 4,000 A. the junction depthof the present invention may be as shallow as 1.500 A. To acquirethisjunction depth of 1.500 A with phosphorus. the phosphorus isdiffused into the p-type substrate at about 750 to 825 C for about 5-l0minutes. The diffusion gas having the impurities comprises 0 N and PH,(source of phosphorus). and is fed into the diffusion furnace at a rateof l.000 cc/min. for N 500 cc/min. of 99% Argon. 171 PH and cc/min. of OThe volume concentration of phosphorus in the surface layer would be ofthe order of magnitude of ID or 10 atoms/cubic centimeter. If arsenic orantimony were used then the time and temperature of diffusion would bechanged as would be known. to acquire a junction depth of 1.500 A.

After diffusion. the n-p silicon material is exposed to steam for about2 minutes at 800C. This results in the formation of 1.000 A of SiO(glass) extending from the top surface of the n-type material. In theoxidation process approximately several hundred (400-500) A of siliconare removed from the top of the diffused layer which results in severaladvantages. First. during the diffusion process approximately 450 A ofthe top surface is damaged which results in a shortening of the lifetimeof electrons approaching the surface. Removal of the 400500 A therebyimproves the lifetime of these electrons. Secondly. removal ofthe 400500A of silicon further reduces the junction depth which means more shortwavelength light will propagatebeyond the junction to generate morecarriers therein.

As an additional step in the process of making n-p solar cells of thepresent invention, all or part of the 1.000 A of the SiO may be removedin a conventional manner. Full or partial removal of the SiO would bedesirable since the glass has an index of refraction of only about 1.46which means too much light will be reflected from the surface of thesolar cell.

The silicon slab is now ready to have the fine geometry pattern placedon the top surface of the diffused layer 2. First. the top surface iscoated completely with a photoresist of any known type. e.g.. A-Zresist. Then the photorcsist is exposed to light or an electron beamthrough any desired mask having a fine pattern such as the fine geometrypattern of FIG. 3. The method of masking a fine lined mask is wellknown. The top surface is then developed with any known developer usedwith the A-Z resist thereby forming the pattern areas (i.e., thefingers) on the bare diffused silicon layer.

Next, using a known vapor deposition technique. about 300 A of chromiumis evaporated on the entire top surface followed by the evaporation of2.000 A of Ag. The photoresist is then dissolved in any known solventused with the A-Z resist. The solvent lifts off the photoresist. and,consequently, the metal on top of the photoresist in areas (between thefingers) where photoresist is in contact with the silicon. This lift offprocess is knownw as lift-off photolithography and results in the finemetallic geometry pattern. i.e.. the finger pattern shown in FIG. 3,positioned on the top of the diffused layer 2. Finally, to build up thethickness of each of the metallic fingers to about microns for purposesof good conductivity. silver is plated, in a known manner, onto themetallic fingers.

An alternative photolithography technique for forming the fine geometrypattern would comprise the ordered steps of (l) evaporating a metal,e.g., chromium, over the top surface; (2) covering the metal withphotoresist; (3) exposing the photoresist to light or an electron beamthrough a mask; (4) developing the photoresist; (5) etching off themetal in the area between the fingers (6) using a solvent to dissolvethe residual photoresist.

The effect of radiation damage to solar cells is reduced with the finegeometry solar cell of the present invention. Radiation damage to asolar cell affects the response of the cell to longer wavelengths. Thepresent invention, by obtaining more energy output from the shortwavelength region than prior art solar cells. has therefore reduced theoverall effect of radiation dam age.

Referring to FIG. 4 there is shown a graph ofthe efficiency of a solarcell with respect to the surface concentration of diffused layer 2. Ascan be seen, the efficiency obtainable with the fine geometry solar cellis approximately 50 percent greater than the efficiency obtainable withthe standard six finger geometry solar cells.

What is claimed is:

l. A solar cell comprising a semiconductor material having top andbottom surfaces and having a p-n junction at a distance of between 500 Aand 2,000 A from the top semiconductor surface thereof, said top surfacebeing adapted to receive incident light radiation, an electrode on saidbottom semiconductor surface, and a patterned electrode on said topsemiconductor surface. said patterned surface comprising a plurality ofthin metallic fingers electrically connected together. said thinmetallic fingers being separated by a [distances distance on the order]of n x i0 centimeters where n is a non-zero integer any mimhvrfmm (my tonine.

2. A solar cell as claimed in claim I wherein said semiconductormaterial is silicon.

3. A solar cell as claimed in claim 2 wherein said pn junction is at adepth of approximately L500 A and divides said semiconductor materialinto a top surface layer and a bottom layer.

4. A solar cell as claimed in claim 2 wherein said thin metallic fingersarc separated by distances of approximately 0.03 centimeters.

S. A solar cell as claimed in claim 2 wherein said p-n junction dividessaid semiconductor material into atop n-typc layer and a bottom p-typclayer.

6. A solar cell as claimed in claim 2 wherein said matcrial between saidtop surface and said pn junction has an impurity concentration of aboutIO' l0 impurity atoms/cm.

7. A solar cell as claimed in claim 2 wherein said thin metallic fingersare spread substantially evenly over the surface of said solar cell at adensity in one dimension of approximately 30 metallic fingers percentimeter.

8. A solar cell as claimed in claim 2 wherein the width of said fingersis between about l20 microns.

9. A solar cell as claimed in claim 3 wherein said thin metallic fingersare separated by distances of approxi' mately 0.03 centimeters.

[0. A solar cell as claimed in claim 3 wherein said p-n junction dividessaid semiconductor material into a top n-type layer and a bottom p-typelayer.

11. A solar cell as claimed in claim 3 wherein said material betweensaid top surface and said p-n junction has an impurity concentration ofabout 10' l0 impurity atoms/cm.

12. A solar cell as claimed in claim 3 wherein said thin metallicfingers are spread substantially evenly over the surface of said solarcell at a density in the dimension of approximately 30 metallic fingersper centimeter.

13. A solar cell as claimed in claim 3 wherein the width of said fingersis between about 1-20 microns.

14. A solar cell as claimed in claim 9 wherein said p-n junction dividessaid semiconductor material into a top n-type layer and a bottom p-typclayer.

15. A solar cell as claimed in claim 14 wherein said material betweensaid top surface and said p-n junction has an impurity concentration ofabout IO' I0 impurity atoms/cm".

[6. A solar cell as claimed in claim 15 wherein said impurity atoms areatoms selected from a group consisting of phosphorous. arsenic andantimony.

17. A solar cell as claimed in claim l6 wherein said thin metallicfingers are spread substantially evenly over the surface of said solarcell at a density in one dimension ofapproximately 30 metallic fingersper centimeter.

18. A solar cell as claimed in claim 17 wherein the width of saidfingers is between about [-20 microns.

UNITED STATES PATENT AND TRADEMARK OFFICE CERTIFICATE OF CORRECTIONPATENT NO. 1 RE 28,610

DATED November 11, 1975 INVENTOR(S) 2 Joseph Lindmayer It is certifiedthat error appears in the aboveidentified patent and that said LettersPatent are hereby corrected as shown below:

IN THE SPECIFICATION:

Column 1, line 12 after "solar", insert cell Column 3,

line 29 after may", insert also Column 4,

line 32 delete "750 to 825" and insert 750 to 825 line 44 delete "800C"and insert 800 C Column 5, line 31 after "fingers", insert a semi-colonIN THE CLAIMS:

Column 6, line 37 after "in", delete "the" and insert one Signed andSealed this twentieth Day Of April 1976 [SEAL] I A Hes t:

RUTH'C. M A SON C. MARSHALL DANN Attestmg ()jjrcer (mnmissimwr ofParents and Trademarks

1. A solar cell comprising a semiconductor material having top andbottom surfaces and having a p-n junction at a distance of between 500 Aand 2,000 A from the top semiconductor surface thereof, said top surfacebeing adapted to receive incident light radiation, an electrode on saidbottom semiconductor surface, and a patterned electrode on said topsemiconductor surface, said patterned surface comprising a plurality ofthin metallic fingers electrically connected together, said thinmetallic fingers being separated by a (distances) distance (on theorder) of n X 10 2 centimeters where n is (a non-zero integer) anynumber from one to nine.
 2. A solar cell as claimed in claim 1 whereinsaid semiconductor material is silicon.
 3. A solar cell as claimed inclaim 2 wherein said p-n junction is at a depth of approximately 1,500 Aand divides said semiconductor material into a top surface layer and abottom layer.
 4. A solar cell as claimed in claim 2 wherein said thinmetallic fingers are separated by distances of approximately 0.03centimeters.
 5. A solar cell as claimed in claim 2 wherein said p-njunction divides said semiconductor material into a top n-type layer anda bottom p-type layer.
 6. A solar cell as claimed in claim 2 whereinsaid material between said top surface and said p-n junction has animpurity concentration of about 1019 - 1020 impurity atoms/cm3.
 7. Asolar cell as claimed in claim 2 wherein said thin metallic fingers arespread substantially evenly over the surface of said solar cell at adensity in one dimension of approximately 30 metallic fingers percentimeter.
 8. A solar cell as claimed in claim 2 wherein the width ofsaid fingers is between about 1-20 microns.
 9. A solar cell as claimedin claim 3 wherein said thin metallic fingers are separated by distancesof approximately 0.03 centimeters.
 10. A solar cell as claimed in claim3 wherein said p-n junction divides said semiconductor material into atop n-type layer and a bottom p-type layer.
 11. A solar cell as claimedin claim 3 wherein said material between said top surface and said p-njunction has an impurity concentration of about 1019 - 1020 impurityatoms/cm3.
 12. A solar cell as claimed in claim 3 wherein said thinmetallic fingers are spread substantially evenly over the surface ofsaid solar cell at a density in the dimension of approximately 30metallic fingers per centimeter.
 13. A solar cell as claimed in claim 3wherein the width of said fingers is between about 1-20 microns.
 14. Asolar cell as claimed in claim 9 wherein said p-n junction divides saidsemiconductor material into a top n-type layer and a bottom p-typelayer.
 15. A solar cell as claimed in claim 14 wherein said materialbetween said top surface and said p-n junction has an impurityconcentration of about 1019 - 1020 impurity atoms/cm3.
 16. A solar cellas claimed in claim 15 wherein said impurity atoms are atoms selectedfrom a group consisting of phosphorous, arsenic and antimony.
 17. Asolar cell as claimed in claim 16 wherein said thin metallic fingers arespread substantially evenly over the surface of said solar cell at adensity in one dimension of approximately 30 metallic fingers percentimeter.
 18. A solar cell as claimed in claim 17 wherein the width ofsaid fingers is between about 1-20 microns.